The present invention relates to a method for purifying aluminum and use thereof. More specifically, the present invention relates to a method for producing industrially useful highly purified aluminum in which the amount of eutectic impurities such as Si and Fe are fully reduced by utilizing a segregation theory, and a use of purified aluminum obtained by said method.
As a method for purifying aluminum utilizing a segregation theory, there are known a method in which molten aluminum portions near the solid-liquid interface are stirred when cooling to crystallize molten aluminum which is a starting material in a vessel, from lower surface or side surface, a method in which aluminum crystallized on the inner wall of a vessel keeping molten aluminum and on the surface of a cooling body immersed in molten aluminum are scraped and compacted at the bottom of the vessel for molten aluminum by a piston and the like a method in which highly purified aluminum is allowed to crystallize on the surface of a cooling body immersed into molten aluminum while revolving the cooling body, and the like.
Japanese latent Application Publication (JP-B) No. 59-41500 discloses a method for producing highly purified aluminum in which molten aluminum to be purified is charged into a crucible equipped with a stirrer, and aluminum is allowed to crystallize and grow while cooling from the bottom of the crucible and revolving and raising the stirrer.
JP-B No. 1-37458 discloses a method for purifying aluminum in which molten aluminum is kept in a vessel which is equipped with a level floor and has a side wall of heat-insulation structure, and an aluminum crystal is allowed to crystallize on the surface of the floor while stirring the molten aluminum by a stirrer inserted in the molten aluminum and on one hand, passing a cooling medium through a cooling medium flowing tube inside the floor, on the other hand, heating the surface of the molten aluminum.
Japanese Patent Application Laid-Open (JP-A) No. 5-125462 discloses a method for purifying aluminum in which molten aluminum starting material is kept in a vessel, and highly purified aluminum is allowed to crystallize on the bottom of the vessel while thermally insulating the upper part and side part of the vessel under controlled temperature condition, cooling the aluminum from the bottom of the vessel, and revolving the vessel around the center vertical axis, and simultaneously, allowing the molten aluminum in the vessel to rotate by a stirrer toward the reverse direction to the revolving direction of the vessel.
JP-A No. 59-170227 discloses a method for purifying aluminum in which a highly purified aluminum crystal is crystallized on the inner wall of a vessel keeping molten aluminum, this crystal is scraped onto the bottom of the vessel, and the stacked crystal is compacted.
JP-A No. 62-158830 discloses a method for purifying aluminum in which an aluminum crystal crystallized on the surface of a cooling body immersed in molten aluminum is compressed to the bottom of the vessel by a piston, and the remaining molten aluminum is discharged.
JP-B No. 61-3385 discloses a method for purifying aluminum in which highly purified aluminum is crystallized on the surface of a cooling body immersed in molten aluminum while revolving the cooling body under specific condition.
However, in these conventional technologies, impurities in the resultant purified aluminum can not necessarily be reduced sufficiently. In these methods for purifying aluminum utilizing a Segregation theory, the proportion of an eutectic impurity which can be reduced in one purification operation is limited.
In purification methods utilizing a segregation theory, the reduction ratio of an impurity can be represented by the purification coefficient (=concentration of impurity element in purified aluminum/concentration of impurity element in starting aluminum). For example, when molten aluminum containing eutectic impurities such as Si, Fe and the like are purified by a purification method utilizing a segregation theory, the purification coefficients of these eutectic elements are less than 1. Accordingly impurities can be reduced, however, it is generally difficult to reduce the purification coefficient lower than the equilibrium distribution coefficient (=equilibrium concentration of impurity element in purified aluminum/equilibrium concentration of impurity element in molten aluminum) represented by phase diagrams of Alxe2x80x94Si, Alxe2x80x94Fe and the like. Actually, it is generally difficult to reduce the purification coefficient lower than the effective distribution coefficient (distribution coefficient in the case when concentration of impurity element increases more than equilibrium concentration at the molten aluminum side of solid-liquid interface) which is larger than the equilibrium distribution coefficient. Further, when aluminum is industrially purified, about 20 to 70% of molten aluminum is allowed to crystallize, and as a result of increase of the concentration of eutectic impurity elements in molten aluminum, the purification coefficient becomes larger and the amount of the impurities in the resultant purified aluminum can not be reduced sufficiently.
As a industrial method 2 or more continuously combined purification operations, a journal xe2x80x9cSHO-ENERGY, vol. 35, no. 4, p.45(1983)xe2x80x9d discloses a method, called Kohzyunal Process.
The object of the present invention is to provide an industrially advantageous method for producing highly purified aluminum in which eutectic impurities such as Si, Fe and the like are sufficiently reduced, and a use of purified aluminum obtained by maid method.
The present inventors have intensively studied a method for purifying aluminum utilizing a segregation theory in view of such conditions, and found an industrially advantageous method for purifying aluminum which can reduce eutectic impurities extremely efficiently by conducting purification process continuously using the specific steps, and have completed the present invention.
Namely, the present invention composed of the following [1] to [15] aspects.
[1] A method for purifying aluminum which comprises steps of;
maintaining aluminum which is a starting material in the form of molten condition in the N+1 vessels at first, wherein N+1 (N is an integer of 2 or more) vessels for molten aluminum and N cooling bodies which highly purified aluminum is crystallized on their surfaces are arranged sequentially, respectively, and
repeating a series of processes (1) to (4) twice or more:
wherein a series of processes (1) to (4) is as follows;
(1) a process in which a n-th (n is an integer from 1 to N) cooling body is immersed into molten aluminum in a n-th vessel and highly purified aluminum is allowed to crystallize on the surface of the cooling body,
(2) a process in which N cooling bodies which highly purified aluminum has been crystallized on their surface are lifted from molten aluminum, then, the vessels and cooling bodies are relatively moved so that a n-th cooling body can be immersed into molten aluminum in a (n+1)-th vessel, further, aluminum which is a starting material is supplied to the first vessel (herein, it is not necessary to supply aluminum in the last repeating cycle of a series of processes (1) to (4)),
(3) a process in which N cooling bodies which highly purified aluminum has been crystallized on their surface are immersed so that a n-th cooling body is immersed into molten aluminum in a (n+1)-th vessel and is heated to melt the highly purified aluminum crystallized on the surface of the cooling body, and
(4) a process in which N cooling bodies which highly purified aluminum crystallized on their surfaces has been molten are lifted from molten aluminum, then, the vessels and A cooling bodies are relatively moved so that a n-th cooling body can be immersed into molten aluminum in a n-th vessel, further, molten aluminum purified from a (N+1)-th vessel is recovered (herein, it is not necessary to recover purified aluminum in the last repeating cycle of a series of processes (1) to (4)).
[2] A method for purifying aluminum which comprises steps of:
maintaining aluminum which is a starting material in the form of molten condition in the N+1 vessels at first, wherein N+1 (N is an integer of 2 or more) vessels for molten aluminum and N cooling bodies which highly purified aluminum is crystallized on their surfaces are arranged sequentially, respectively,
repeating a series of processes (1) to (4) twice or more,
discharging molten aluminum in the first vessel,
supplying a part of purified aluminum recovered in the process (4) in the form of molten condition into the first vessel,
maintaining aluminum in completion of the preceding cycle in the form of molten condition in the N+1 vessels in the second and following cycles, wherein the first cycle is completed by arranging the first vessel as a (N+1)-th vessel and arranging (n+1)-th (n is an integer from 1 to N) vessel which keeps molten aluminum as a n-th vessel, and
conducting the same procedure as in the first cycle except maintaining aluminum which is a starting material in the form of molten condition in the N+1 vessels at first:
wherein a series of processes (1) to (4) is as follows:
(1) a process in which a n-th cooling body is immersed into molten aluminum in a n-th vessel and highly purified aluminum is allowed to crystallize on the surface of the cooling body,
(2) a process in which N cooling bodies which highly purified aluminum has been crystallized on their surface are lifted from molten aluminum, then, the vessels and cooling bodies are relatively moved so that a n-th cooling body can be immersed into molten aluminum in a (n+1)-th vessel, further, aluminum which is a starting material is supplied to the first vessel (herein, it is not necessary to supply aluminum in the last repeating cycle of a series of processes (1) to (4)),
(3) a process in which N cooling bodies which highly purified aluminum has been crystallized on their surface are immersed so that a n-th cooling body is immersed into molten aluminum in a (n+1)-th vessel and is heated to melt the highly purified aluminum crystallized on the surface of the cooling body, and
(4) a process in which N cooling bodies which highly purified aluminum crystallized on their surfaces has been molten are lifted from molten aluminum, then, the vessels and cooling bodies are relatively moved so that a n-th cooling body can be immersed into molten aluminum in a n-th vessel, further, molten aluminum purified from a (N+1)-th vessel is recovered (herein, it is not necessary to recover purified aluminum in the last repeating cycle of a series of processes (1) to (4)).
[3] The method according to [2], wherein the N+1 vessels are linearly arranged sequentially from 1-st to (N+1)-th and in completion of the cycle, the first vessel is moved to the most rear part of the (N+1)-th vessel.
[4] The method for purifying aluminum according to [2], wherein the N+1 vessels are circularly arranged at a constant interval sequentially from 1-st to (N+1)-th and in completion of the cycle, all of the vessels are revolved by 360/(N+1) degree around the center of the circle.
[5] The method for purifying aluminum which comprises steps of:
maintaining aluminum which is a starting material in the form of molten condition in the N+1 vessels,
wherein N+1 (N is an integer of 2 or more) vessels for molten aluminum and N cooling bodies which highly purified aluminum is crystallized on their surfaces are arranged sequentially, respectively,
repeating a series of processes (1) to (4) twice or more,
discharging molten aluminum in the first vessel,
transferring molten aluminum in a (n+1)-th (n is an integer from 1 to N) vessel to a n-th vessel sequentially,
wherein a part of purified aluminum recovered in the process (4) is supplied in the form of molten condition into the (N+1)-th vessel and the first cycle is completed,
maintaining aluminum in completion of the first cycle in the form of molten condition in the N+1 vessels in the second and the following cycle, and
conducting the same procedure as in the first cycle except maintaining aluminum which is a starting material in the form of molten condition in the N+1 vessels at first;
wherein a series of processes (1) to (4) is as follows:
(1) a process in which a n-th cooling body is immersed into molten aluminum in a n-th vessel and highly purified aluminum is allowed to crystallize on the surface of the cooling body,
(2) a process in which N cooling bodies which highly purified aluminum has been crystallized on their surface are lifted from molten aluminum, then, the vessels and cooling bodies are relatively moved so that a n-th cooling body can be immersed into molten aluminum in a (n+1)-th vessel, further, aluminum which is a starting material is supplied to the first vessel (herein, it is not necessary to supply aluminum in the last repeating cycle of a series of processes (1) to (4)),
(3) a process in which N cooling bodies which highly purified aluminum has been crystallized on their surface are immersed so teat a n-th cooling body is immersed into molten aluminum in a (n+1)-th vessel and is heated to melt the highly purified aluminum crystallized on the surface of the cooling body, and
(4) a process in which N cooling bodies which highly purified aluminum crystallized on their surfaces has been molten are lifted from molten aluminum, then, the vessels and cooling bodies are relatively moved so that a n-th cooling body can be immersed into molten aluminum in a n-th vessel, further, molten aluminum purified from a (N+1)-th vessel is recovered (herein, it is not necessary to recover purified aluminum in the last repeating cycle of a series of processes (1) to (4)).
[6] The method for purifying aluminum according to [1], wherein N is 2 or 3.
[7] The method for purifying aluminum according to [1], wherein aluminum is kept in the form of molten condition in N+1 vessels at first, wherein the purity of the aluminum is higher than that of aluminum which is a starting material and the purity of aluminum in a (n+1)-th vessel is higher than that of aluminum in a n-th vessel.
[8] The method for purifying aluminum according to [1], wherein a series of processes (1) to (4) are repeated 5 to 15 times in one cycle.
[9] The method for purifying aluminum according to claim 1, wherein, in the process (1), a process in which molten aluminum in the first vessel is discharged and aluminum which is a starting material is supplied in the form of molten condition to the first vessel one or twice in one cycle before a n-th cooling body is immersed in molten aluminum in a n-th vessel.
[10] The method for purifying aluminum according to [1], wherein, in the process (1), a n-th cooling body is immersed in molten aluminum in a n-th vessel, and the molten aluminum is allowed to rotate around the center of the cooling body and a gas which forms gas bubble in the molten aluminum is introduced into the molten aluminum to crystallize highly purified aluminum on the surface of the cooling body.
[11] The method for purifying aluminum according to [10], wherein the gas which forms gas bubble is air.
[12] The method for purifying aluminum according to [1], wherein, in the process (1), a n-th cooling body is immersed in molten aluminum in a n-th vessel, and the cooling body is allowed to revolve to crystallize highly purified aluminum on the surface of the cooling body.
[13] The method for purifying aluminum according to [1], wherein, in the process (1), a n-th cooling body having a periphery temperature of less than the melting point of aluminum is immersed in molten aluminum in a n-th vessel while the n-th cooling body is revolved, or, in the process (2), N cooling bodies which highly purified aluminum has been crystallized on their surface are lifted from molten aluminum while N cooling bodies are revolved, or, a n-th cooling body having a periphery temperature of less than the melting point of aluminum is immersed in molten aluminum in a n-th vessel while the n-th cooling body is revolved in the process (1) and N cooling bodies which highly purified aluminum has been crystallized on their surface are lifted from molten aluminum while N cooling bodies are revolved in the process (2).
[14] The method for purifying aluminum according to [1], wherein, in the whole process, the purification coefficients of Si and Fe (=concentration of impurity element in purified aluminum/concentration of impurity element in starting aluminum) are 0.1 to 0.05, respectively, and the product yield (=amount of recovered product/amount of charged starting material) is over 0.4.
[15] An aluminum foil for an electrolytic capacitor produced by using purified aluminum as a starting material obtained by the method according to [1].